Direct Synthesis of Novel Cu2-xSe Wurtzite Phase Emil A. Hernández-Pagán, ∫∆‡† Evan H. Robinson, ∫∆‡ Andrew D. La Croix,∫ ∆† and Janet E. Macdon- ald*∫∆

∫Department of Chemistry, ∆The Vanderbilt Institute of Nanoscale Science and Engineering, Vanderbilt University, Nash- ville, Tennessee 37235, United States *Email: [email protected] KEYWORDS: phase control, nanocrystals, copper , wurtzite, organochalcogenide

ABSTRACT: Herein, we report the unprecedented direct synthesis of a recently discovered metastable wurtzite phase of Cu2-xSe. Nanocrystals of Cu2-xSe were synthesized employing dodecyl diselenide as the source and ligand. Optical characterization performed with UV-vis-NIR spectroscopy in solution showed a broad plasmonic band in the NIR. Structural characterization was performed with XRD and TEM. Variable temperature XRD analysis revealed the wurtzite nanocrystals irreversibly transform into the thermodynamic cubic phase at 151°C. Replacement of dodecyl diselenide with dodecyl selenol yielded cubic phase Cu2-xSe, allowing for polymorphic phase control. An aliquot study was performed in order to gain insight into the mechanism of phase selec- tivity. The direct synthesis of this novel wurtzite phase could enable the discovery of new phenomena and expand the vast application space of CuxSey compounds.

1. INTRODUCTION There have been a few literature reports of direct synthetic The phase of a material is critical in determining the proper- control of the of Cu2Se. Liu and coworkers de- ties exhibited. For example, wurtzite and rock salt CdSe have a veloped a synthetic scheme in which Cu reacted with 1 TOP:Se yielded tetragonal or cubic phase Cu2Se based on the direct and indirect , respectively. Similarly, PbO-type 18 FeSe exhibits while the NiAs-type does not.2 reaction temperature. Low et al. employed a solventless ther- However, phase control in the synthesis of polymorphic sys- molysis of a copper(I) phenylselenoate polymer under vacuum or inert atmosphere to obtain orthorhombic or cubic Cu2Se, re- tems, in particular access to metastable phases, remains a chal- 23 lenge and is the focus of intense research efforts.3–10 The Cu spectively. However, the direct synthesis of the hexagonal 2- phase has remained elusive because current reaction schemes xSe (x=0-0.2) system is of particular interest because this p-type direct band gap can exhibit plasmonic proper- rely on Se precursors, such as TOP:Se, ODE:Se, and ties, superionic conductivity and low thermal conductivity OLAM:Se, that require temperatures ≥ 180 °C to decompose. based on composition.11–14 These characteristics make this ma- The higher temperatures likely facilitate formation of the ther- terial ideal for applications in solar energy, bioimaging,15 opto- modynamic phase. The use of diorganyl dichalcogenides in electronics,13,16 and thermoelectrics.17 nanocrystal synthesis has gained momentum in recent years and notably often at more moderate synthetic temperatures.5,10,16,24– Controlling the structure of Cu2-xSe (x=0-0.2) in synthesis 28 In particular, the Brutchey group has exploited these synthons presents a challenge because the Cu-Se phase diagram is com- to yield the metastable hexagonal wurtzite phases of CuInS2, positionally rich and additionally includes Cu3Se2, CuSe, and 24 CuInSe2, and Cu2SnSe3. Herein, we report the direct synthesis CuSe2. Furthermore, bulk and nanoscale Cu2-xSe have been re- and characterization of metastable hexagonal wurtzite Cu2-xSe ported to crystallize in monoclinic, tetragonal, orthorhombic, employing didodecyl diselenide (DD Se ) as the Se precursor. and cubic crystal systems.12,14,17–19 Recently Gariano et al. syn- 2 2 thesized hexagonal Cu2Se with the wurtzite phase via cation ex- change of wurtzite-CdSe with Cu+.20 While there is very little known about this novel phase, the anisotropic nature of the crys- 2. EXPERIMENTAL SECTION tal system and the high mobility of Cu+ in copper chalcogenides 2.1 Materials. Selenium powder (≥99.5 trace metals basis), 1-bro- could enable the discovery of new phenomena and the utiliza- mododecane (97%), formic acid (≥95% reagent grade), anhydrous tion in applications like those mentioned above. Additionally, N,N-dimethylformamide (DMF, 99.8%), tetrahydrofuran (THF, wurtzite Cu Se can serve as a host material in cation exchange ≥99.9%), and 1-octadecene (ODE, 90% technical grade) were obtained 2-x from Sigma Aldrich. Sodium borohydride and hydrochloric acid (ACS reactions to access metastable phases of other with Plus) were obtained from Fisher Chemicals. Cu(acac)2 (≥98%) was unique properties.21,22

obtained from Strem Chemicals. All materials were used as received ray diffractometer with a CuKα (λ = 0.154 nm) radiation source set to without additional purification. 40 kV and 44 mA, and a D/teX Ultra 250 1D strip detector. 2.2. Ligand synthesis. Synthesis of Didodecyl Diselenide: XRD patterns were acquired using a step size of 0.1 degrees at 1 or 10 C24H50Se2 was synthesized following a previously reported proce- degrees per minute. High temperature XRD measurements were per- dure.29 A 1L three-neck round bottom flask was flushed with argon formed using a Rigaku Multipurpose High Temperature Attachment and maintained under inert atmosphere throughout. 75 ml of deionized and a PTC-EVO temperature controller, with a ramp rate of 2 degrees 1 H2O was added, followed by 9.3 g (0.12 mol) of selenium powder. The per minute and a holding time of 2 min. H NMR spectra were taken reaction was stirred and cooled on an ice bath for 10 min. 9.8 g (0.26 using a Bruker DRX-400 (tuned to 400 MHz) spectrometer. Spectra 13 mol) of NaBH4 was added portion-wise so that the evolution of hydro- were calibrated to residual solvent signals of 7.26 ppm in CDCl3. C gen gas did not cause a rapid increase in temperature. The reaction was and 77Se NMR spectra were taken using a Bruker DRX-500 (tuned to then allowed to stir for 20 min. Another 9.3 g (0.12 mol) of selenium 100 MHz or 76 MHz for 13C or 77Se, respectively) spectrometer. 13C powder was added and the reaction was allowed to cool to room tem- spectra were calibrated to residual solvent signals of 77.0 ppm in perature followed by an additional 20 min. of stirring at room temper- CDCl3. ature. The flask was then heated to 70 °C and temperature maintained 2.6 Computational Methods. The computational methods per- for 20 min. resulting in a clear dark red solution. The reaction was formed herein were adapted from the works of Guo et al.28 and Rhodes allowed to cool to room temperature. 1-bromododecane (58.6 g, 0.24 et al.10 Bond dissociation energies were calculated using Gaussian with mol) and 280 ml of tetrahydrofuran were added dropwise over 30 min. density functional theory (DFT) and the Boese-Martin Kinetics (BMK) under vigorous stirring. The reaction was then allowed to react at 50 functional. Molecular geometry optimization was performed using the °C for 18 hours, and then it was cooled to room temperature. The 6-31G(d) basis set and single-point energy calculations were performed phases were separated. The organic layer was washed once with de- using the 6-311G(d,p) basis set. ionized H2O, then combined organic layers were washed with brine and dried over MgSO4. The solvent was removed under reduced pressure. 3. RESULTS AND DISCUSSION The crude produce was recrystallized from heptane and isopropyl alco- hol over an ice bath to yield yellow needles. Yield: 53.5%. In a typical synthesis, Cu(acac)2 and DD2Se2 were mixed in 1 H NMR (400MHz, CDCl3) δ 2.92 (t, 4H, J=7.6 Hz), 1.73 (quint, 4H, octadecene and heated to 155 °C for 1 hr (see experimental sec- J=7.5 Hz), 1.27 (m, 36H), 0.88 (t, 3H, J=7.1 Hz).13C NMR (100 MHz, tion for details). The reaction mixture was cooled down to room CDCl3) δ 31.86, 30.93, 30.20, 29.61, 29.58, 29.55, 29.47, 29.30, 29.10, temperature and the nanocrystals were separated by centrifuga- 77 22.63, 14.04. Se NMR (76 MHz, CDCl3) δ 307.7. tion and purified by washing with acetone. The reaction tem- Synthesis of Dodecyl Selenol: C12H25SeH was synthesized follow- perature was selected based on an observed change from a tur- ing a previously reported procedure.29 A 250 mL three-neck round bot- bid mixture to an optically clear solution indicative of nanocrys- tom flask was flushed with argon and maintained under inert atmos- tal nucleation. Figure 1 illustrates the powder XRD pattern of phere throughout. To the flask was added selenium powder (4.6 g, 0.06 mol) and sodium borohydride powder (4.5 g, 0.12 mol). Anhydrous the as-synthesized nanocrystals. The diffraction peaks match ethanol (21 ml) as added slowly to the solution. With 20 min. of stir- that of the calculated pattern reported by Gariano et al. that ring, a white/grey solid gradually formed. Anhydrous DMF (100 ml) was slowly added. The reaction turned a rufous color, then gradually turned clear and colorless over 30 min. Formic acid (4.6 ml, 0.125 mol) was then added dropwise and the reaction was allowed to stir for 20 min. 1-bromododecane (12 ml, 0.05 mol) was added slowly to the mix- ture and the reaction was allowed to stir for 4-6 hours, dependent on completion of reaction as verified by quenched aliquots with TLC. (Note: failure to allow reaction to run to completion results in a mixture of dodecylselenol and dodecyl bromide, which forms an azeotrope dur- ing distillation and is otherwise challenging to separate by other com- mon purification methods). The reaction was then hydrolyzed with 100 ml of 10% HCl. 50 ml of H2O was added to the mixture, and the or- ganic was extracted 3x with 50ml of Et2O. The combined organics were washed once with 10% HCl and dried over MgSO4. The solvent was removed under reduced pressure. Vacuum distillation produced 1 pure dodecyl selenol. H NMR (400MHz, CDCl3) δ 2.58 (q, 2H, J=7 Hz), 1.69 (quint, 2H, J=7.5 Hz), 1.26 (m, 18H), 0.88 (t, 3H, J=7.0 Hz), 13 -0.70 (t, 1H, J=6.8 Hz). C NMR (100 MHz, CDCl3) δ 33.95, 31.85, 29.58, 29.57, 29.54, 29.49, 29.45, 29.28, 28.92, 22.62, 17.63, 14.04. 77 Se NMR (76 MHz, CDCl3) δ -13.1. 2.3 Nanocrystal syntheses. Synthesis of hexagonal Cu2-xSe NCs: Standard Schlenk line techniques were used throughout with N2 as the inert gas. In a typical synthesis, 1.1 mmol of didodecyl diselenide, 0.5 mmol of Cu(acac)2, and 5 mL of octadecene were mixed in a 25 mL 3- necked flask. The flask was put under vacuum and the temperature was raised to 80 ˚C for 30 min. Then the flask was put under inert atmos- phere and the temperature was raised to 155 ˚C for 1 hour. The flask Figure 1. Powder XRD of the wurtzite Cu2-xSe nanocrystals (top), was allowed to cool to room temperature, and the post-reaction mixture with unit cell and the reference pattern obtained from Gariano, et was precipitated in acetone three times and resuspended in chloroform. al., cubic (ICSD: 181661), tetragonal (JCPDS: 29-0575), mono- Synthesis of cubic Cu2-xSe NCs: The same procedure as described for clinic (JCPDS: 27-1131), (ICSD: 16949) and klockman- the hexagonal Cu2-xSe NCs was followed except didodecyl selenol was nite (ICSD: 82331) phases of . used in place of didodecyl diselenide. 2.4 Instrumentation. Transmission Electron Microscopy (TEM) was performed on a FEI Technai Osiris digital 200 kV S/TEM system. Solution absorption spectra were obtained on a Jasco V-670 UV-Vis- NIR spectrophotometer in tetrachloroethylene. X-ray diffraction (XRD) measurements were done using a Rigaku SmartLab powder X- corresponds to metastable wurtzite Cu2-xSe (space group Cu2Se, in agreement with the EDS results (Table S1). Further- 20 P63mc), not those of cubic Cu2-xSe, tetragonal and monoclinic more, the relatively slow formation of a NIR surface plasmon Cu2Se, tetragonal umangite (Cu3Se2) and hexagonal klockman- and its position indicate that our synthetic conditions and ligand nite (CuSe) (Figure 1). Moreover, quantitative energy-disper- environment yield nanocrystals that are only weakly susceptible sive X-ray spectroscopy (EDS) provided a composition close to to oxidation. It should be noted the wurtzite Cu2-xSe NCs re- Cu2Se (Table S1). mained in the hexagonal crystal structure and with similar mor- Transmission Electron Microscopy (TEM) analysis revealed the Cu2-xSe nanocrystals have a disk morphology (Figure 2) with an average diameter of 13.8 ± 1.0 nm. Further examination with high-resolution TEM (HR-TEM) and fast Fourier trans- form (FFT) of the images (Figures S1A-B) reveal an anisotropic crystal structure with the hexagonally close-packed layers stacked along the c-axis. The selected area electron diffraction (SAED) in Figure S2 is consistent with the XRD data. Based on the bulk Cu-Se phase diagram, berzelianite (cubic)

Figure 3. UV-vis-NIR spectra of wurtzite Cu2-xSe nanocrystals after the work up and after one week of storage, both under ambient con- ditions.

Figure 2. TEM images of Cu2-xSe nanocrystals. A. Disk-shaped NCs aligned in rows. Inset: hexagonal morphology highlighted phology after weeks of air exposure (Figure S3 and S4). with close-packed NCs. In light of the prolonged stability of the hexagonal phase of the nanocrystals, variable temperature XRD was employed to Cu2-xSe (0 < x ≤ 0.2) is the thermodynamic phase in the copper- force a phase transition at elevated temperatures. Data in Figure rich region at temperatures below 1000 K.30 Although the ef- 4A shows XRD patterns collected from 37 ˚C to 186 ˚C with fects of surface energy and ligands in nanoscale materials can some key features. Diagnostic of the phase change, the reflec- alter the phase diagram, it has been shown that synthesized tion corresponding to the (111) plane of berzelianite Cu2-xSe monoclinic and tetragonal Cu2Se nanocrystals transform to ber- emerges as the temperature increases at the expense of the 12,14 zelianite Cu2-xSe upon exposure to air. In those systems, ox- (002), (100) and (101) reflections of the wurtzite structure. Fol- idation of copper and conversion to the thermodynamic sub- lowing the work of Rivest et al.,33 phase transition temperature stoichiometric Cu2-xSe phase are closely related and occur in was calculated by fitting a Boltzmann function to the integrated concert. Copper migration from the NC core to the surface ac- intensity of the diminishing wurtzite peak at 25.2˚. As depicted counts for the stoichiometric “loss” of oxidized Cu in the cubic in Figure 4B, the transition temperature from the wurtzite to phase (however, the precise location of Cu2+ may not be known, berzelianite crystal structure was found to be approximately 151 and could remain in the core after oxidation31). This oxidation ˚C. To verify the transition temperature, the synthesized nano- results in an increase of charge carrier density giving rise to a crystals were heated in octadecene at 150 ˚C for 1 hr; as was plasmon band in the NIR region.12,14,31 The synthesized nano- crystals were purified under ambient conditions, thus one could expect to see this plasmon band. Optical characterization was performed by UV-Vis-NIR spectroscopy in solution. The absorbance spectrum is shown in Figure 3. Interestingly, the absorbance spectrum does not ex- hibit a significant plasmon band in the NIR region. One week of exposure to ambient conditions was required for a broad plas- mon band centered at 1900 nm to develop. This is in contrast to observations by Dorfs et al. where cubic Cu1.96Se nanocrystals exhibited a plasmon band at ~ 1600 nm after one minute under ambient conditions.32 Similarly, Kriegel et al. observed the emergence of an intense plasmon band in the spectral region ca 1300-1400 nm as tetragonal Cu2Se nanocrystals oxidized to cu- 14 bic Cu1.8Se within 5.5 hrs. The red shifted plasmon band ob- served here suggests a composition close to stoichiometry expected the nanocrystals transformed into the berzelianite (cu- bic) phase (Figure S5). The transition temperature being close to the synthesis tem- perature highlights the advantage of regents active at moderate temperatures. Reaction aliquots taken below 155°C showed no evidence of nanocrystal formation, yet a distinct color change from blue to brown was observed at 155°C as nanocrystals formed. Control syntheses performed at 165 ˚C and 175 ˚C yielded predominantly wurtzite nanocrystals but with phase im- purities (Figure S6). At 185 ˚C, berzelianite Cu2-xSe nanocrys- tals were obtained. It appears the reactivity of DD2Se2 is at a bare minimum to achieve the phase purity of the metastable phase observed. A reagent any less reactive and requiring higher reaction temperatures or longer times would have re- sulted in significant impurities of the thermodynamic cubic phase. Given this low transition temperature, the direct synthesis of wurtzite Cu2-xSe was in part achievable because of the moderate reaction temperature facilitated by the facile decomposition of DD2Se2. A reagent any less reactive and requiring higher reac- tion temperatures would have resulted in significant impurities of the thermodynamic cubic phase. To test if the lower reaction temperature was the only factor responsible for the formation of the wurtzite phase, DD2Se2 was replaced with dodecylselenol (DDSeH). When DDSeH was used under otherwise identical conditions, the reaction yields the thermodynamic product, berzelianite Cu2-xSe (Figure S7) in- stead of the wurtzite product. Previous literature reports have shown control of the product phase when using dialkyl dichal- cogenides with different substituents and with dialkyl dichalco- genides vs alkyl thiol/selenol.10,24 The phase control has been attributed in part to the relative strength of the C-X and X-X Figure 4. A. Variable temperature powder XRD of wurtzite Cu2-xSe (X=Se, S) bonds, and consequently the chalcogenide transfer nanocrystals along with the reference patterns for wurtzite and ber- rate. More easily cleaved bonds in the organochalcogenide lead zelianite Cu2-xSe. Platinum reflections at 39.8˚ and 46.2˚ due to the to the thermodynamic products. For example, in the synthesis sample holder have been omitted. B. The cubic 25.2˚ reflections 25 of CuInSe2, Tappan et al. were able to selectively obtain the were integrated and plotted versus temperature to calculate the thermodynamic chalcopyrite or the metastable wurtzite phase transition temperature of 151 ˚C per Rivest et al. CuInSe2 product employing dibenzyl diselenide (C-Se 43 or dimethyl diselenide (weaker C-Se bonds) were employed. kcal/mol) or diphenyl diselenide (C-Se 65 kcal/mol), respec- Conversely, reactions involving diphenyl diselenide (stronger tively, as the Se source. Similarly, the copper selenide interme- C-Se bonds) progressed via the metastable intermediate uman- diates of these reactions showed a similar trend. The reaction gite Cu Se . progressed through the thermodynamically stable intermediates 3 2 DFT calculations (Figure S8) revealed C-Se bond strengths of berzelianite Cu2-xSe and klockmannite CuSe when dibenzyl of 55.4 kcal/mol and 67.8 kcal/mol for DD2Se2 and DDSeH,

Figure 5. Aliquot XRD study of the precursor formation during Cu2-xSe syntheses. A. When dodecaneselenol is used in a NC synthesis, the selenol reacts with Cu early on in the heat-up to reaction temperature of 155 ˚C, generating a lamellar mesophase similar to that observed by Bryks, et al. results with Cu2S. B. Didodecane diselenide shows less reactivity during the heat-up and only Cu(acac)2 is detected with XRD. respectively, following the same trend as previously reported Ligands may also alter the surface energies of the growing nu- for other organochalcogenides. Given the above hypothesis, clei,37 and our group has previously shown that thiol sulfur pre- one would expect the DD2Se2 to give the thermodynamic phase cursors, can provide highly bridged surfaces chemistries of the and DDSeH to give the metastable phase, yet the results here thiol ligands.39,40 Further work in the corresponding orga- are the opposite. Therefore, other factors are at play. noselenide chemistry is in progress. To shed some light on the mechanistic details that lead to phase control, aliquots of the reactions taken at 80 °C, 120 °C, 4. CONCLUSION 0 min@155 °C, and 5 min@155 °C were analyzed with XRD. In summation, a direct synthesis for the recently discovered Figure 5A shows the patterns for the syntheses with DDSeH. copper(I) selenide wurtzite phase was developed using di- The XRD pattern for the 80 °C aliquot reveals the formation of dodecyl diselenide as a nanocrystal selenium source and ligand. XRD confirmed the hexagonal phase initially discovered by a Cu-selenoate complex (CH3(CH2)10CH2Se-Cu). This complex was also observed in the work of Bryks et al.27 and Berends et Gariano, et al., and TEM revealed a platelet-like morphology. al.26 This Cu-selenoate complex is a lamellar mesophase analo- A high-temperature study showed that the metastable wurtzite gous to the well documented metal-thiolate complexes, which phase converted to the thermodynamic berzelianite phase at ap- have been exploited in the synthesis of metal and metal sulfide proximately 151 ˚C. DDSeH alternatively provided the cubic nanocrystals.34–36 The reflections corresponding to the complex Cu2Se product, through a copper selenoate intermediate with a remained present at 120 °C, yet subsided significantly when the hypothesized weaker C-Se bond than DD2Se2. The synthesis of system reached the reaction temperature (0 min@155 °C). After other metal selenides with DD2Se2 and DDSeH as ligands/sele- nium source is currently in progress, as well as an investigation 5 min at 155 °C, the complex was no longer present, indicative into how the additional ligands (e.g. TOP) alter the resulting of the nucleation onset of Cu Se. In contrast, for DD Se (Fig- 2-x 2 2 phase as previously observed by others. ure 5b), the XRD patterns of the four corresponding aliquots early in the reaction sequence matched the reflections for Cu(acac)2. The presence of the diselenide bond prevents for- mation of the Cu-selenoate complex as presumably a homolytic ASSOCIATED CONTENT of the Se-Se bond would be required as opposed to a simple ligand exchange. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. In the context of the previous observations of others of phase control based on bond strength of the precursors, we posit that Additional characterization: XRD, TEM, SAED, EDS. (PDF) the formation of the complex results in Cu withdrawing electron density from Se, weakening the C-Se bond for the DDSeH. Since the copper-thiolate complex does not readily form as an AUTHOR INFORMATION intermediate for DD2Se2, this reagent is the less reactive of the two. Thus, in agreement with the previous observations in the Corresponding Author literature, the more-reactive precursor leads to the thermody- * [email protected] namically favored cubic phase under synthetic conditions. Present Addresses It is known that ligands can direct the phase of the products 9,10,37 † E.A.H.-P. current address: 104 Chemistry Building, Penn State in other metal-chalcogenide systems, and the copper sele- University, University Park, PA 16801 USA. nide system is similarly susceptible. As mentioned earlier, a A.D.L. current address: Department of Chemistry, Lander Cu-selenoate complex was observed to form in the work by 27 26 University, 320 Stanley Avenue, Greenwood, SC 29649 Bryks et al. and Berends et al., however these two reactions USA yielded different products. The solventless thermolysis of a Cu- selenoate complex performed by Bryks et al. formed the ther- Author Contributions modynamic product, berzelianite Cu2-xSe. This system is simi- lar to ours, but without the non-coordinating solvent octadecene ‡These authors contributed equally. present in our reaction, which facilitates the diffusion of reac- tants. In contrast, the reaction system employed by Berends et Note al. contained additional ligands (e.g. trioctylphosphine oxide, The authors declare no competing financial interest. oleylamine) and lead to nanocrystals of umangite Cu3Se2. Sim- ilarly, Tappan et al. observed umangite Cu3Se2 en route to ACKNOWLEDGMENT wurtzite CuInSe2 in the presence of oleic acid and oleylamine. Care must therefore be taken when comparing results with dif- This work was financially supported by NSF grant EPS-1004083 fering ligands present. The reaction system in the present work and the Vanderbilt Institute of Nanoscale Science and Engineering. was intentionally simplified by utilizing a non-coordinating sol- EHR thanks and acknowledges support from the Mitchum E. War- vent, and DD Se or DDSeH as both the selenium source and ren Fellowship. The authors thank Ross Koby and Jordan Rhodes 2 2 for assistance with DFT calculations. ligands. Ligands may play unrecognized roles in the decomposition ABBREVIATIONS mechanism of the organochalcogenide reactants. Our lab has DD Se , didodecyl diselenide; DDSeH, dodecylselenol shown, for example, that oleylamine plays a key role in the de- 2 2 10 composition of allyl disulfide. 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